1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating a micromachine package.
2. Description of the Related Art
Micromachine sensing elements (hereinafter micromachine elements) are well known. A micromachine element typically includes a miniature moveable structure, such as a bridge, cantilevered beam, suspended mass, membrane or capacitive element, which is supported over a cavity formed in a silicon wafer. Since the operation of the micromachine element depends upon the moveability of the miniature moveable structure, it is critical that the package, which includes the micromachine element, does not contact the miniature moveable structure in any manner.
FIG. 1 is a cross-sectional view of a structure 8 during the formation of a plurality of micromachine packages in accordance with the prior art. As shown in FIG. 1, a silicon wafer 10 included a plurality of micromachine chips 12. Micromachine chips 12 included micromachine areas 14 on a front surface 10F of wafer 10. Micromachine areas 14 included the miniature moveable structure of the micromachine element. Micromachine chips 12 further included bond pads 16 on front surface 10F of wafer 10. Bond pads 16 were connected to the internal circuitry of micromachine chips 12.
Micromachine chips 12 were often integrally connected together in an array. Each of micromachine chips 12 in the array was delineated by a singulation street 20, which was located between adjacent micromachine chips 12.
A lid 30 was positioned above wafer 10. Lid 30 included a plurality of caps 42 integrally connected to one another. Each cap 42 included a micromachine cavity 32. Each micromachine cavity 32 was positioned over a corresponding micromachine area 14. Generally, micromachine cavities 32 were wider than micromachine areas 14.
Each cap 42 further included a bond pad cavity 34. Each bond pad cavity 34 was positioned over a corresponding set of bond pads 16 on a micromachine chip 12. Generally, bond pad cavities 34 were wider than bond pads 16, and were at least as deep as bond pads 16 were tall.
FIG. 2A is a cross-sectional view of structure 8 of FIG. 1 at a further stage in fabrication in accordance with the prior art. As shown in FIG. 2A, lid 30 was attached to wafer 10. Micromachine cavities 32 were positioned above corresponding micromachine areas 14. Further, bond pad cavities 34 were positioned above corresponding sets of bond pads 16.
FIG. 2B is a cross-sectional view of structure 8 of FIG. 2A at a further stage of fabrication in accordance with the prior art. Referring to FIG. 2B, a series of shallow cuts were made to remove a portion of each cap 42 to expose bond pads 16. Micromachine chips 12 were electrically tested by connecting test probes to bond pads 16. If testing of a micromachine chip 12 indicated that the micromachine chip 12 was defective, the micromachine chip 12 and/or corresponding cap 42 was marked. For example, micromachine chip 12A was marked as being defective. Wafer 10 was then singulated along singulation streets 20. Micromachine chips 12 which were marked as defective were discarded.
Disadvantageously, a cap 42 was attached to a micromachine chip 12 even if the micromachine chip 12 was defective. The cap 42 and defective micromachine chip 12 were discarded. However, since a cap 42 was attached to the defective micromachine chip 12, the cost associated with the defective micromachine chip 12 was increased compared to the cost associated with the defective micromachine chip 12 alone. This increased the cost of fabricating each batch of micromachine packages. This, in turn, increased the average total cost of fabricating each individual micromachine package, which passed testing.
After singulation of wafer 10, each good micromachine chip 12 with cap 42 was further packaged. FIG. 3 is a cross-sectional view of a single micromachine package 40 in accordance with the prior art. As shown in FIG. 3, micromachine chip 12 and cap 42 were attached to a substrate 52. Bond pads 16 were electrically connected to traces 44 by bond wires 46. To prevent accumulation of static charge on cap 42, which would render micromachine chip 12 inoperable, cap 42 was electrically connected to a ground trace 48 by a bond wire 50. Ground trace 48 was grounded during use. Although effective at prevent accumulation of static charge on cap 42, grounding cap 42 by electrically connecting cap 42 to ground through bond wire 50 and ground trace 48 was relatively labor intensive and complex which increased the cost of fabricating package 40.